Achromatic focusing effect of metasurface-based binary phase Fresnel zone plate

Traditional binary phase Fresnel zone plate (BPFZP) fails to provide a single-layered solution for multi-wavelength achromatic focusing. In this article, by combining metasurfaces and BPFZP model, we propose an ultra-thin single-layered transmissive achromatic phase Fresnel zone plate (PFZP) with large numerical aperture (NA) working in the visible range that is hardly possible using traditional Fresnel zone plate. We present designs of both monochromatic and achromatic BPFZP displaying almost equal simplicity despite lens size or NA while possessing moderate focusing efficiency attributed to the BPFZP model. The achromatic BPFZP designed consists of periodic cylindrical TiO₂ nanopillars with thickness of 375 nm, holds NA of 0.97 and experiences average focal distance deviation of 8%. The circularly-symmetric composing antennas also provide the achromatic BPFZP with polarization-independence performance. This design method with certain error robustness makes it feasible for large-scale fabrication and compact integration with CCD/CMOS or fiber devices for future multi-wavelength applications with high resolution.